Cassegraninian antenna having limited spillover energy

ABSTRACT

A deflector is fixed relative to coaxial main and secondary antenna reflectors, after said secondary reflector and between the ground and the axis of the antenna to intercept parasitic radiation spillover.

United States Patent Tocquec et al.

[451 Dec. 19, 1972 CASSEGRANINIAN ANTENNA HAVING LIMITED SPILLOVER ENERGY Inventors: Yves Tocquec, Marcoussis; Andre Rabadeux, Palaiseau, both of France lndustrielle Des Cit-Alcetel,

Campagnle Telecommunications Paris, France Filed: Aug. 25, 1971 Appl. No.: 174,600

Assignee:

Foreign Application Priority Data Aug. 28, 1970 France "7031536 US. Cl ..343/7s2, 343/837 1m. Cl. ..l-l01q 19/14 [58] Field 01 Search ..343/781, 782, 833, 837

[56] References Cited UNITED STATES PATENTS 3,209,361 9/1965 Webb, ..343/781 3,430,244 2/1969 Bartlett et a1 ..343/781 X 3,530,476 9/1970 Ravenscroft ..343/837 X Primary Examiner-Stanley D. Miller, Jr. Attorney-Richard C. Sughrue et al.

[5 7] ABSTRACT A deflector is fixed relative to coaxial main and secondary antenna reflectors, after said secondary reflector and between the ground and the axis of the antenna to intercept parasitic radiation spillover.

4 Claims, 2 Drawing Figures l x 2 1 a" l I I l I CASSEGRANINIAN ANTENNA HAVING LIMITED SPILLOVER ENERGY BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention concerns a Cassegrainian antenna which is so improved that its spillover energy is limited.

2. DESCRIPTION OF THE PRIOR ART Cassegrainian antennas for electromagnetic waves are known, which consist of a main reflector, which may be parabolic or pseudo-parabolic, of a pseudohyperbolic or elliptical secondary reflector and of a source of excitation whose transmission or reception pattern is directed towards the secondary reflector.

It is known that an important factor in the impairment of the performances of antennas of this type is the loss due to spillover energy. The transmission energy does not entirely reach'the secondary reflector and a part may fall to the ground.

The harmful effect of thisspillover energy causes a deterioration of the gain of the antenna and a lowering of the quality factor which depends upon this gain and upon the noise temperature of the ground and of the sky. The noise temperature of the ground is constant and high. That of the sky, which is variable, depends upon the angle of elevation of the antenna system in relation to the ground. The temperature of the sky increases as the angle of elevation decreases. Nevertheless, at frequencies of the order of 5 Gc/s and at elevations as low as 5, the temperature of the ground is greater than that of the sky.

It is mainly at low elevations that the coupling of the spillover energy with the ground is maximum.

The method generally employed to combat this coupling consists in covering the ground around the antenna by a metallic grating. This reflecting grating, which forms a screen, stops the radiation from the ground in the direction of the antenna and the spillover portion of the lobe of this antenna is sent back to the sky, where the luminance temperatures are low. However, the grating can be employed only with antennas of small dimensions, because the surface to be covered is then only of a few square meters. Moreover, the grating system is applicable only at low frequencies, because the meshes can be loose at these temperatures. In the case of large antennas operating at high frequencies, the cost of such gratings and the space occupied thereby become prohibitive, it being necessary for the grating to extend over lengths of hundreds of meters and for the meshes to be close together so as to exhibit a spacing of the order of a centimeter.

SUMMARY OF THE INVENTION The arrangement according to the invention makes it possible to obviate these disadvantages, since it is possible therein to reduce the coupling between the spillover energy and the ground by means of a simple system occupying a small amount of space.

The invention relates to a Cassegrainian antenna for microwave electromagnetic radiation, comprising, in this order, a main reflector, a source of excitation which transmits or receives a radiation lobe, and a secondary reflector coaxial with the said main reflector and receiving on its reflecting face a portion of the said radiation lobe, a spillover portion of the lobe being directed, in certain low positions, from the axis of the antenna in the direction of the ground and thus producing the parasitic spillover coupling through the ground, characterized by the fact that the antenna comprises in addition an electrically conductive deflector which is fast with the said secondary reflector, the said deflector being disposed after the said secondary reflector, on the side remote from he reflecting face of the said secondary reflector, in the space between the ground and the said axis of the antenna, in such manner as to intercept the said spillover portion.

In accordance with one feature of the invention, the antenna is characterized by the fact that the said deflector comprises a plane surface which is at an angle of about 45 to the said axis of the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the diagrammatic figures, there will now be described an example of the application of the present invention, this example being intended purely for illustration and having no limiting character. In these two figures, like elements are denoted by like references throughout.

FIG. 1 is a geometrical optical diagram of a Cassegrainian antenna provided with a deflector according to the invention.

FIG. 2 is a diagrammatic view in perspective of a secondary reflector of a Cassegrainian antenna and of the deflector according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, there will be seen meridians of the main reflector 1 and of the secondary reflector 2 of a Cassegrainian antenna. The main reflector is a paraboloid about 30 meters in diameter and the secondary reflector is a hyperboloid whose diameter is one-tenth as large. The two reflectors are of like focus: F1 represents the focus of the paraboloid and one of the foci of the hyperboloid, and F2 represents the other focus of the hyperboloid. The transmission source of the antenna is disposed at F2. The electromagnetic energy in the form of a lobe (not shown) is radiated from the source in the direction of the secondary reflector 2. A portion of the spillover energy is not intercepted by the secondary reflector 2. A deflector 3 performing the function of a plane mirror is disposed at the level of the secondary reflector 2. The plane of the deflector 3 forms with the axis of symmetry 4 of the Cassegrainian antenna an angle of about 45. In accordance with the laws of geometrical optics, a ray 5 emanating from the source is first reflected by the secondary reflector 2 and then by the principal reflector 1. This ray can thereafter take a direction parallel to the axis 4 of the antenna. Conversely, when the antenna operates in reception, the path traveled by the ray is the same but of opposite direction.

In the case where a radiated energy spillover is considered, the ray 6 not passing through the secondary reflector 2 and directed in the direction of the ground 7 is intercepted by the deflector 3. The ray 6 is thereafter reflected towards the sky. Coupling of the spillover energy with the ground is thereby avoided. It may be observed that the deflector according to the invention is particularly advantageous at low elevations at the anl060ll 0249 tenna, i.e. at the angles formed by the axis of symmetry 4 with the plane of the ground 7 between 5 and The deflector 3 is disposed between the ground 7 and the axis of symmetry 4 of the antenna. In order to avoid an increase of the masking effect caused by the secondary reflector and in order to permit ready fixing, the deflector 3 is set back from the secondary reflector 2.

The trace of the plane of the deflector 3 does not extend beyond the extension of the meridian of the secondary reflector 2.

In FIG. 2, there is shown a perspective view of the secondary reflector 2 and of the deflector 3. The deflector 3 is of substantially square form, the dimensions'bein'g approximately equal to the diameter of the secondary reflector 2. A trapezoidal recess 7 opens into the interior of the square surface of the deflector 3, which consists of a metallic plate made of an alloy of aluminum, zinc and magnesium known under the trade name AG 5. Owing to the recess 7, the weight of the deflector may be reduced to about 100 kg.

The recess 7 is masked by the secondary reflector 2 from the rays coming from the source. This recess does not interfere with the reflection of the rays 6 corresponding to the spillover energy directed directed towards the ground.

The plate of the deflector 3 is secured by means of a I series of bolts such as 9 and 10 to a right-angled member 8 attached to beams (not shown). In order to counteract the forces due to the wind, rods 11 and 12 connected to two ends of the plate of the deflector 3 abut two points of the right-angled member 8. Likewise, two other ends of the plate of the deflector 3 are connected by rods 13 and 14 to beams (not shown) which support the secondary reflector 2.

Although the arrangement just described appears to be the most advantageous, it will be appreciated that various modifications may be made thereto without departing from the scope of the invention, it being possible for some of the elements of the arrangement to be replaced by others capable of performing the same technical function therein.

The arrangement according to the invention may be employed in all cases where a Cassegrainian antenna operating at low elevations produces a coupling of the spillover energy with the ground.

Particularly interesting applications may be in the field of telecommunications by satellites in the band from 4 to 6 Gc/s.

What is claimed is:

1. In a Cassegrainian antenna for microwave electromagnetic radiation, including, in this order, a main reflector, a source of excitation which transmits or receives a radiation lobe, a secondary reflector coaxial with the said main reflector and receiving on its reflecting face a part of the said radiation lobe, a spillover portion of the lobe being directed, in certain low positions, from the axis of the antenna in the direction of the ground, thus producing the parasitic spillover coupling through the ground, the improvement wherein the antenna comprises in addition an electrically conductive deflector fixed relative to said secondary reflector, said deflector being disposed after the said secondary reflector, on the side remote from the reflecting face of the said secondary reflector in the s ace situated betw en the ground and the said axis of t e antenna, in suc manner as to intercept the said spillover portion.

2. The antenna according to claim 1, wherein said deflector comprises a plane surface which forms an angle of about 45 with said axis of the antenna.

3. The antenna according to claim 2, wherein the plane surface of the deflector has an approximately square form within which lies a trapezoidal recess.

4. The antenna according to claim 2, wherein the overall dimensions of said plane surface are substantially equal to the diameter of said secondary reflector.

* s: a a

l060ll 0250 

1. In a Cassegrainian antenna for microwave electromagnetic radiation, including, in this order, a main reflector, a source of excitation which transmits or receives a radiation lobe, a secondary reflector coaxial with the said main reflector and receiving on its reflecting face a part of the said radiation lobe, a spillover portion of the lobe being directed, in certain low positions, from the axis of the antenna in the direction of the ground, thus producing the parasitic spillover coupling through the ground, the improvement wherein the antenna comprises in addition an electrically conductive deflector fixed relative to said secondary reflector, said deflector being disposed after the said secondary reflector, on the side remote from the reflecting face of the said secondary reflector in the space situated between the ground and the said axis of the antenna, in such manner as to intercept the said spillover portion.
 2. The antenna according to claim 1, wherein said deflector comprises a plane surface which forms an angle of about 45* with said axis of the antenna.
 3. The antenna according to claim 2, wherein the plane surface of the deflector has an approximately square form within which lies a trapezoidal recess.
 4. The antenna according to claim 2, wherein the overall dimensions of said plane surface are substantially equal to the diameter of said secondary reflector. 